1. Sockets and URLs
3-Dec-18

2. Sockets
A socket is a low-level software device for connecting two computers together
Sockets can also be used to connect two programs running on the same computer
There are two kinds of sockets:
A Socket is a client socket, and initiates the communication
A ServerSocket waits for a request; it then
May perform the requested action
May return a result to the requester

3. Socket constructors import java.net.*;
Socket(InetAddress address, int port)
Creates a client socket and connects it to the specified port number at the specified IP address
ServerSocket(int port)
Creates a server socket on the specified port, with a default queue length of 50
The actual work, for both kinds of sockets, is done by an instance of the SocketImpl class

4. Some Socket methods getInputStream() getOutputStream() close()
Returns an input stream for this socket
getOutputStream()
Returns an output stream for this socket
close()
Closes this socket

5. Some ServerSocket methods
accept()
Listens for a connection to be made to this socket and accepts it
Returns a Socket as its result
Once you have a Socket , you can use the same methods on both the client and the server sides
For input, use the getInputStream() and getOutputStream() methods on the returned Socket
close()
Closes this socket

6. Socket fields
protected InetAddress address
The IP address of the remote end of this socket
protected FileDescriptor fd
The file descriptor object for this socket
protected int localport
The local port number to which this socket is connected
protected int port
The port number on the remote host to which this socket is connected
All of these except FileDescriptor have associated getter methods

7. URLs A URL is a Uniform Resource Locator, typically a Web address
Some constructors:
URL(String spec)
Creates a URL object from the information in the String
URL(String protocol, String host, int port, String file)
Creates a URL object from the specified protocol, host, port number, and file
The host may be localhost, to indicate “this machine”
URL(String protocol, String host, String file)
Same as above, but uses the default port for this protocol

8. Some URL methods openStream() openConnection()
Opens a connection to this URL and returns an InputStream for reading from that connection
openConnection()
Returns a URLConnection object that represents a connection to the remote object referred to by the URL
getProtocol(), getHost(), getPort(), getFile(), getQuery()

9. Using a URLConnection The steps in using a URLConnection are:
Use openConnection() to create the URLConnection object
Set the parameters as desired:
setAllowUserInteraction, setDoInput, setDoOutput, setIfModifiedSince, setUseCaches, setRequestProperty
Use the connect method to make the actual connection
Use the remote object

10. Some URLConnection methods
getHeaderField(int n)
Returns the value for the nth header field
getHeaderField(String name)
Returns the value of the named header field
getHeaderFieldKey(int n)
Returns the key for the nth header field
getInputStream()
Returns an input stream that reads from this open connection
getOutputStream()
Returns an output stream that writes to this connection

11. The End